Sen. Robert Menendez surprised no one Tuesday when he became the second Democratic senator to oppose the Iran deal, but the momentum is clearly in favor of the deal among Democrats.

"Unlike President Obama's characterization of those who have raised serious questions about the agreement, or who have opposed it, I did not vote for the war in Iraq, I opposed it, unlike the vice president and the secretary of State, who both supported it," the New Jersey Democrat said. "My vote against the Iraq war was unpopular at the time, but it was one of the best decisions I have ever made."

He characterized his view as a principled disagreement, highlighting statistics from CQ Roll Call that show him voting with President Barack Obama's positions 98 percent of the time over the past two years.

"I know that, in many respects, it would be far easier to support this deal, as it would have been to vote for the war in Iraq at the time. But I didn't choose the easier path then, and I’m not going to now. I know that the editorial pages that support the agreement would be far kinder, if I voted yes, but they largely also supported the agreement that brought us a nuclear North Korea," Menendez said in remarks at Seton Hall University.

Menendez, a former chairman of the Foreign Relations Committee, has consistently crossed party lines to lead efforts to impose additional sanctions on Iran in recent years, as well as working on the legislation that ultimately provided for September's floor debate on disapproving of the nuclear agreement.

Menendez told the audience at Seton Hall that the way the deal developed, he had regrets about not moving ahead with another round of biting sanctions he drafted with Sen. Mark S. Kirk, R-Ill.

But as of Tuesday, other than Menendez, only New York Democratic Sen. Charles E. Schumer has come out in opposition to the agreement among members of their party in the Senate. There's been a steady pattern of Democratic caucus members coming out in support, 21 of the 46 to date.

"A rejection by the U.S. of this deal will not bring our partners back to negotiate another deal. In fact, at a recent meeting of leaders from our partner nations, I specifically asked the Ambassadors to the U.S. from China, the United Kingdom, and Russia whether their countries would come back to negotiate again should the U.S. walk away from the deal. They unanimously said, 'No', that there was already a deal — the one before Congress," Hirono said in a statement.

"I came to these negotiations with an open mind. Prioritizing engagement over coercion in an attempt to end three decades of animosity with Iran appeals to the American idealism in us all. And while we should strongly support diplomacy, the other side must believe there are real consequences in its failure. In this case, Iran never felt that, resulting in a very disappointing outcome for our country," Corker wrote. "Throughout history, Congress has rejected or altered hundreds of international agreements, many of them multilateral. For the administration to say there is no other deal than this one is an effort to negate Congress’s important role and responsibility."

But on Monday, the Associated Press reported that Majority Leader Mitch McConnell, R-Ky., said in the town of Georgetown in his home state that Obama and deal supporters had "a great likelihood of success." That's in no small part because the president only needs one more than one-third of either chamber of Congress to sustain a veto.

"I hope we can defeat it, but the procedure is obviously stacked in the president's favor," McConnell said.

McConnell said he anticipated the GOP would unite in opposition to the agreement.